Kapustin and Witten showed that a twisted version of $N=4$ gauge theory in four dimensions compactifies to a two-dimensional sigma-model whose target space is the Hitchin moduli space. In this talk, I consider the reduction of the gauge theory on a four dimensional orientable spacetime manifold which is not a global product of two surfaces but contains embedded non-orientable surfaces. The low energy theory is a sigma-model on a two dimensional worldsheet whose boundary components end on branes constructed from the Hitchin moduli space associated to a non-orientable surface. I will also compare the discrete topological fluxes in four and two dimensional theories and verify the mirror symmetry on branes as predicted by the $S$-duality in gauge theory. This provides another non-trivial test of $S$-duality using reduction along possibly non-orientable surfaces. Finally, I consider the quantisation of the Hitchin moduli space from a non-orientable surface as an example of quantisation via branes and mirror symmetry.